The invention relates to a flat gasket for sealing a sealing gap between sealing surfaces of machine components which can be pressed against the flat gasket. The flat gasket has a gasket plate comprising at least two sheet metal layers arranged on one another.
The sheet metal layers forming the two main surfaces of the gasket plate of such flat gaskets are usually provided with beads at particularly critical places with respect to the sealing function where an increased sealing surface pressure is to be generated between the flat gasket and the sealing surfaces of the machine components which are to be sealed off from one another. However, such beads have proven inadequate at places where the machine component sealing surfaces, between which the flat gasket is clamped, are uneven, for example, have steps, as is the case when with a machine component sealing surface resting against the flat gasket there are several, for example, two, sealing surface areas which are formed by several separate machine components resting against one another in the area of this step, i.e., forming a jointxe2x80x94at such joints manufacturing and/or assembly tolerances can result in a step in the said sealing surface.
This problem typically occurs with flat gaskets in the form of cylinder head gaskets for reciprocating internal combustion engines with an, in particular, multipart chain case which is likewise to be sealed off by the cylinder head gasket, because in this case the two machine component sealing surfaces between which the cylinder head gasket is clamped, are formed, on the one hand, by the cylinder head and a chain case top part, and, on the other hand, by the engine block or crankcase and a chain case bottom partxe2x80x94with such an engine construction there is both a joint between the cylinder head and the chain case top part and a joint between the engine block and the chain case bottom part. However, the top part of the chain case can also be formed by the cylinder head so that there is no separate chain case top part, or the bottom part of the chain case can be formed by the engine block so that there is no separate chain case bottom part. Nevertheless, a joint still results in the bottom or top one of the sealing surfaces between which the cylinder head gasket is clamped.
Several solutions to the above-described problem caused by joints are to be found in the prior art:
EP-A-0 059 777 discloses a single-layer, metallic flat gasket for a multipart gear housing with a housing top part and a housing bottom part, between which the flat gasket is clamped. The housing top part itself is made up of several parts so that joints result in the sealing surface of the housing top part which has to be pressed against the flat gasket, and these can lead to steps in an area of the sealing surface. For this reason, the flat gasket has an elongate, slot-shaped opening, which extends over all these joints when the gasket is installed and into which a strand-shaped, elastomeric sealing element is placed, which projects on both sides above the two main surfaces of the flat gasket and is made as a prefabricated insert.
Solutions to the above-described problem for cylinder head gaskets which also serve to seal off a chain case are also to be found in the prior art:
DE-C-43 37 758 shows a multilayer, metallic cylinder head gasket in which prefabricated elastomeric sealing elements are inserted, more specifically, at places on the cylinder head gasket which each lie over a joint between engine block and chain case. In a first, four-layered embodiment of this cylinder head gasket, each of the sealing elements has an approximately T-shaped cross section with a sealing portion and a head portion which is thickened in relation to the latter and serves to fix the sealing element in the cylinder head gasket. For this purpose, the two center layers have for each sealing element rectangular, window-like openings lying above one another for receiving the head portion of the respective sealing element, while the one cover or outer layer is closed in the area of the sealing elements and the other cover layer has for each sealing element a rectangular, window-like opening through which the sealing portion of the sealing element engages and which is smaller than the openings of the center layers and the cross section of the head portion. In the unpressed state of the cylinder head gasket, the sealing portions of the sealing elements already project to a considerable extent beyond the cover layer provided with the openings. In a second embodiment of this known cylinder head gasket, which has three layers, all three layers have at the locations of the sealing elements rectangular, window-like openings lying above one another, and the openings of the center layer are somewhat larger than the openings of the cover layers, which are of the same size, so that the sealing elements are held securely against displacements between the cover layers with central areas of increased diameter. In this second embodiment, too, the sealing elements already project beyond the one main surface of the gasket in the unpressed state of the cylinder head gasket.
Finally, EP-B-0 701 051 discloses a multilayer, metallic cylinder head gasket which also serves to seal off the chain case. The gasket plate of this cylinder head gasket has a relatively thick center layer and two relatively thin outer layers, which are each provided with a so-called half bead, which extends around the chain case opening as a bead line closed within itself (in contrast to full beads of approximately U-shaped cross section a half bead has a cross section which corresponds to a step or Z pulled more or less flat). Beside each of the two narrow sides of the chain case opening the center layer has a cutout in the form of an elongate, rectangular window, which extends parallel to the adjacent narrow side of the chain case opening, and a prefabricated, strand-shaped, elastomeric sealing element which is inserted with play in the respective window and projects on both sides over the two main surfaces of the center layer (already prior to installation of the cylinder head gasket, i.e., while it is unpressed). One of the outer layers is closed over the sealing elements, i.e., continuously, while the other outer layer has a large, approximately rectangular window which is larger than the chain case opening and exposes the two sealing elements, i.e., in a plan view of the cylinder head gasket, the two sealing elements lie within this window. EP-B-0 701 051 does not explain how with this known cylinder head gasket the two sealing elements are to be prevented from falling out of the gasket during handling thereof. Furthermore, this known cylinder head gasket is only able to partly solve the problem explained at the outset when in the machine component sealing surfaces to be pressed against the cylinder head gasket, there are joints on both sides of the cylinder head gasket because the chain case is comprised of a chain case top part and a chain case bottom part. Moreover, as will be apparent from the following, this known gasket can also lead to problems when on the side of the outer layer provided with the windows there is a chain case part which is inserted from the side during assembly thereof.
When assembling engines with a chain case comprised of a top part and a bottom part, it has already been attempted to solve the problem set forth at the outset by providing the cylinder head gasket on either side of the chain case opening with a relatively small, oval hole which passes through all layers of the multilayer metallic cylinder head gasket and into which an initially viscous sealant is injected in the course of assembly of the engine, which can be cured to an elastomeric material at elevated temperatures, for example, when the engine is first put into operation. It is, however, obvious that such a procedure when assembling the engine in series production is unsatisfactory for several reasons: Today, vehicle manufacturers tend to delegate as many work steps as possible to their suppliers, and in series production it is also difficult, but at least time-consuming to always inject the amount of sealant necessary for reliable sealing into the said holes.
Furthermore, with the flat gaskets in question there is often also a further problem which can be illustrated well with reference to an engine with a multipart chain case: If, for example, the chain case top part is guided parallel to the plane of the sealing gap which is to be sealed by the cylinder head gasket, there is the risk when assembling the chain case top part that an elastomeric sealing element projecting above the surface contour of the gasket plate will be cut off. The same applies accordingly to a chain case bottom part guided parallel to the plane of the sealing gap or to constructions with only one separate chain case part which is guided parallel to the plane of the sealing gap.
The object underlying the invention was to create a flat gasket with a gasket plate which comprises several sheet metal layers, and which is provided with at least one elastomeric sealing element in a gasket plate area which serves to seal a sealing gap area between sealing surfaces of machine components which can be pressed against the flat gasket and in which the component sealing surfaces result in a locally varying sealing surface pressure at the flat gasket, in order to reliably seal the sealing gap also at places where a step may occur in a machine component sealing surface. The flat gasket is to be of such a kind that the sealing element is already applied by the gasket manufacturer and is not subjected to the danger of being damaged or even cut off when assembling the assembly group comprising the machine components and the flat gasket. In particular, the invention is to create a cylinder head gasket for an engine with a chain case, wherein the chain case comprises a chain case top part and/or a chain case bottom part, which contributes or contribute to the formation of the sealing gap to be sealed by the cylinder head gasket.
This object is accomplished by a flat gasket as defined in claim 1, i.e., by a flat gasket for sealing a sealing gap between sealing surfaces of machine components which are pressable against the flat gasket, the flat gasket comprising a gasket plate formed by at least two sheet metal layers, the gasket plate being provided with at least one elastomeric sealing element in a gasket plate area serving to seal a sealing gap area in which the component sealing surfaces lead to a locally varying sealing surface pressure at the flat gasket, the sealing element being arranged on a first one of the sheet metal layers and, in the mounted state of the flat gasket, extending through a cutout in the second sheet metal layer forming a first one of the main surfaces of the gasket plate as far as the component sealing surface adjacent to the second sheet metal layer, and the surface contour of the second sheet metal layer defining at this first main surface of the gasket plate a plane of the maximum level of the said surface contour parallel to the plane of the gasket plate in the unmounted, unpressed state of the gasket. Such a flat gasket according to the invention exhibits the combination of the following features:
(a) the sealing element is in the form of a sealing element formed from a material which is applied to the first sheet metal layer and is curable in situ;
(b) the material of the sealing element is initially capable of undergoing plastic deformation at least at operating temperature (possibly also below it) of the machine components and the flat gasket; and
(c) the gasket plate and the sealing element are so designed that in the unmounted, unpressed state of the flat gasket, the sealing element extends at the most as far as the said plane of the maximum level.
With a flat gasket wherein the second sheet metal layer forming one of the main surfaces of the gasket plate has in the area or in the vicinity of the sealing element (in a plan view of the flat gasket) for example, a bead which forms in the said main surface of the gasket plate an outwardly projecting step or elevation, the said plane of the maximum level is that plane which extends parallel to the plane of the gasket plate and lies on the bead. The same applies accordingly to other elevations of the surface contour of the aforementioned first main surface of the gasket plate in the area or in the vicinity of the sealing element. In a flat gasket according to the invention, the sealing element is thus protected from the danger of being severed, for example, in the course of assembly of the chain case top part and/or chain case bottom part because the sealing element, before the flat gasket is clamped in the sealing gap between the machine components (for example, by tightening the cylinder head screws), does in no way project beyond the said plane of the maximum level, and, consequently, is protected by other gasket parts or by the surface contour of the gasket against damage.
Furthermore, the sealing element can already be mounted by the gasket manufacturer so that when assembling the machine components, no further work need be carried out on the flat gasket itself.
When in the foregoing mention has been made of the fact that the material of the sealing element is to be initially capable of undergoing plastic deformation at least at the operating temperature of the machine components and the flat gasket, the following is to be understood by this: The initially applied, not yet cured material from which the sealing element is formed, should at least also be capable of undergoing plastic deformation and to a certain extent at least until the flat gasket is clamped between the machine components to be sealed off from one another. The material from which the sealing element is formed can already be partially cross-linked prior to installation of the gasket, in order to bring about a certain dimensional stability, without the sealing element thereby losing all plasticity or all ability to flow. The material of the sealing element can then cure fully, for example, at operating temperatures (e.g. of the engine) or at lower temperatures which are elevated with respect to room temperature to the extent that it then only exhibits elastic, but no longer any plastic properties.
The underlying principle of the invention enables the gasket plate and the sealing element to be so designed that a sealing area of the sealing element, which is sealingly effective when the gasket is mounted, is only pressed through the cutout of the second sheet metal layer and pressed against the component sealing surface adjacent to this second sheet metal layer by clamping the flat gasket between the machine components. With the gasket not yet mounted, i.e., unpressed, the sealing element can either not yet project into the cutout of the second sheet metal layer at all or can engage in this cutout or can even engage through this cutout, but at least not project out of this cutout to the extent that the said sealing area of the sealing element projects beyond the above-defined plane of the maximum level of the unpressed gasket.
If a flat gasket according to the invention is provided for an installation situation in which the machine component sealing surfaces lying on both sides of the flat gasket both have steps or similar unevennesses which are difficult to seal off, it is recommended to so design the gasket that also the first sheet metal layer has in the area of the sealing element a further cutout for entry of the sealing element material into this cutout. Firstly, this further cutout can serve to improve the anchoring of the sealing element on the first sheet metal layer. Secondly, or alternatively, the sealing element material can be pressed through this further cutout and pressed against the component sealing surface adjacent to the first sheet metal layer by clamping the gasket in the sealing gap which is to be sealed. If a flat gasket according to the invention requires three or more sheet metal layers, as is often the case with cylinder head gaskets, the principle underlying the present invention opens up several possibilities also in the event that the component sealing surfaces lying on both sides of the flat gasket both have steps or unevennesses which are similarly difficult to seal: One could provide the first sheet metal layer on both sides thereof with sealing elements which pass through cutouts in the outer or cover layers of the gasket when clamping the gasket. Alternatively, in an embodiment in which also the first sheet metal layer has in the area of the sealing element a further cutout for entry of the sealing element material into this cutout, the gasket plate can have on the side of the first sheet metal layer located opposite the second sheet metal layer a third sheet metal layer which is provided over the cutout of the first sheet metal layer with a further cutout for entry and passage of the sealing element material into and through this further cutout.
As will be apparent from the above, the sealing problem with engines having at least one separate chain case part as explained above can be solved with a flat gasket according to the invention, more specifically, with a gasket as defined in claim 8, wherein the cutout of the second sheet metal layer is designed and arranged so as to bridge the joint between a first chain case part and the engine component adjacent thereto (the first chain case part can be a chain case top part or a chain case bottom part and the adjacent engine component can be the cylinder head and the engine block, respectively). If the joint problem explained above exists on both sides of the cylinder head gasket, a cylinder head gasket according to the invention is then so designed that the above-mentioned further cutout (in the first or third sheet metal layer) is designed and arranged so as to bridge the joint between the second chain case part and the engine component adjacent to thereto.
In preferred embodiments of a cylinder head gasket according to the invention for engines with a chain case, the second sheet metal layer is provided in the area or in the immediate vicinity of its cutout with a bead (full bead, but preferably half bead), which forms on the main surface of the gasket plate a projection rising above the cutout of the second sheet metal layer. With such a naturally elastically deformable bead, sealing can be effected with sufficient reliability all around the chain case opening where the engine component sealing surfaces do not have any steps. In addition, or, as an alternative, the bead can protect the sealing element or sealing elements against damage.